Tests around ZoneInfoNotFound are fragile

[pganssle]

The way the pure Python and C extensions are loaded side-by side is somewhat fragile, and actually had to be changed in upstream because the C extension lives at _zoneinfo instead of zoneinfo._czoneinfo (which changes how sys.modules gets populated, evidently).

The most common symptom is that in the test for bad keys, the assertRaises context manager fails to catch ZoneInfoNotFound in one or the other of the modules, because py_zoneinfo.ZoneInfoNotFound gets raised from c_zoneinfo or vice versa, for some reason.

I recall there was also a situation where trying to include typing_example in the tests to run would cause this, and I'm also seeing another manifestation at work.

This probably comes down to something like:

1. The fact that we're doing a partial import (since to import zoneinfo._czoneinfo you need to import zoneinfo first, which then tries to import zoneinfo._czoneinfo).
2. The fact that we're not blocking anything when importing zoneinfo multiple times — one or more of the modules might be getting _tzpath from the other one?

I'll try to come up with a minimal reproducer of at least one of the issues so we can try and work through why it's so flaky.